Firefly and PC GAMESS-related discussion club

>I wanted to set up a run using a 100% PBE exchange and 100% PBE correlation functional; that is; an X+C pairing akin to the option available in GAMESS US 2013 (PBE) and Gaussian 09 (PBEPBE).

>Question: Since Firefly is vastly superior to the mentioned programs, how can I do this in Firefly? Thanks so much!

>Supp. Info: Although the question deals also on "how to generally pair X+C functionals in Firefly", the problem or confusion of not being able to compute 100% CPBE and 100% XPBE with Firefly originates since (1) The DFTTYP=PBE96 or even CPBE in Firefly FF8.0.1.pdf is written as using *also* the "PW91 local correlation" and (2) 21 benchmarks employing Gaussian and Firefly match *perfectly* when calculated with B3LYP but not when calculated with PBEPBE and PBE96, respectively.

>For point (1), one might speculate that since CPBE is actually a "gradient correction", it makes sense to think as CPW91 as the left hand side of Eq. 3 in Phys. Rev. Lett. 77, 3867, 1996. However, this is not clear for the non-expert and in view of the many benchmarks done I appreciate the guidance!

PBE correlation is a sum of local part and non-local part. The local part of PBE correlation in its original form is a PW91L one. The non-local part of PBE correlation depends on the density gradients and on the local part of correlation.

With Firefly, PBE96 functional consists of XPBE96 and CPBE96. Here XPBE96 is PBE 96 exchange functional and CPBE96 is PBE 96 correlation functional. The latter includes local part (PW91L) and non-local part of PBE 96 correlation, so your speculation is absolutely correct.

This detailed partitioning makes sense as one could use e.g. VWN1 or VWN5 local functionals in conjunction with gradient corrected part of PBE 96 correlation.

As to arbitrary X+C pairing, current versions of Firefly do not support this. As to PBEPBE you are right you need to use the predefined PBE96 functional name. The subtle difference in computed energies of Firefly's PBE96 and Gaussian's PBEPBE is most likely explained by the larger sensitivity of PBE correlation energy to the parameters of a grid as compared with B3LYP. In addition, Firefly uses parameters of PW91L functional that are computed to the full decimal precision while other programs may use their approximate values.